Any benefit to adding 5-inch media filter when new system is installed?

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yuup. they are excellent, and they clean the entire duct system, not just the equipment.
I've been selling them for years. I love them. I use the honeywell series fc40

Correction; filter grills "keep" entire duct system from getting dirty....IF THE DUCT SYSTEM IS COMPLETELY SEALED OF ALL AIR INFILTRATION POINTS!

Two things that must be considered when using filter grills:

Duct system needs to be sealed, air tight, throughout the entire system between the filter grill and the system blower. This means that every connection, screw hole etc. must be either sealed with a sealant or taped...or both, to prevent ANY amount of air being drawn into the system without having to pass through the filter.

A filter grill must be larger than a filter installed at the air handler/furnace. The static pressure drop of the ducting and the free area space of the filter grill must be taken into consideration for proper filtration. The surface area of a typical filter grill needs to be about 40% larger than that of a filter located at the blower source.

FIRST and most important fact is __ NEVER USE FILTRETE ! or any filter more than MERV 7 which creates an initial dP >0.2".
You're asking for problems created by very low air flow in many/most situations ! !!

FIRST and most important fact is __ NEVER USE FILTRETE ! or any filter more than MERV 7 which creates an initial dP >0.2".
You're asking for problems created by very low air flow in many/most situations ! !!

5 inch filter should last AT Least a Year.

Higher MERV rated filters are fine as long as they have enough surface area to not cause issues with air flow. The most restrictive filter "can" be just fine as long as it is large enough. Unfortunately, with some of these filters on the market, they would have to be five times as large as they are to not cause air flow issues.

How long a filter lasts is completely up to the conditions of each individual home. A filter that barely gets dirty after a year in one home can very well be needed to be changed within a month in another home. With 5" filters, I always suggest checking them quarterly for the first year to determine how well they are going to do over time.

Another note on filter grilles. Returns need to be ducted to use a filter grille. Loose framed chases or wall cavities are a bad choice for a filter grille. I usually move the filter to the unit when I run across these situations.

I run Aprilaire media filters on both of my units. I have sheading dogs who love to lay near the return. 2-3 months is about it here.

The customer was worried about pet air getting into the newly installed and sealed duct from the filter grille to the furnace media filter and didn't want the expense of 3 fc40s so we sized it accordingly and did it that way.